Touch panel devices are sometimes used as means for inputting into computer systems in FA equipment, OA equipment, measuring equipment and so on. Touch panel devices are generally provided on the display of the equipment, and are for detecting the position where a finger or the like has contacted the surface of the display. Prescribed processing is carried out in the computer system of the equipment based on data relating to images displayed on the display and data relating to the position of contact detected by the touch panel device.
In the technical field of touch panel devices, in recent years SAW type touch panel devices that detect the position of contact using surface acoustic waves (SAWs) have garnered attention. A SAW type touch panel device comprises, for example, a transparent substrate having thereon a detection region and a peripheral region surrounding the detection region, and a plurality of exciting means and a plurality of receiving means provided in the peripheral region on the substrate. Each of the exciting means and the receiving means comprises a piezoelectric device. Such SAW type touch panel devices are described, for example, in Japanese Patent Application Laid-open No. 6-149459 and Japanese Patent Application Laid-open No. 10-55240.
The piezoelectric device constituting each exciting means or receiving means comprises, for example, an interdigital transducer (IDT) formed by patterning in the peripheral region on the substrate for each device, and a piezoelectric film provided in the peripheral region so as to cover the IDT. The IDT comprises a pair of comb tooth electrodes, with each of the comb tooth electrodes having a plurality of mutually parallel electrode fingers. The electrode fingers of one comb tooth electrode and the electrode fingers of the other comb tooth electrode are arranged alternately and parallel to each another. The piezoelectric film is made of a piezoelectric material that exhibits a property of an electric field being produced upon strain being applied thereto (a piezoelectric effect), and a property of strain being produced upon an electric field being applied thereto (an inverse piezoelectric effect).
When an AC voltage is applied to the IDT of a piezoelectric device acting as exciting means, an AC electric field is produced between adjacent electrode fingers. As a result, strain is produced in the piezoelectric film between the electrode fingers through an inverse piezoelectric effect, and hence prescribed acoustic waves are excited in the piezoelectric film by the IDT as a whole. At this time, acoustic waves of a wavelength equal to the pitch of the electrode fingers are excited most strongly. The excited acoustic waves propagate through the surface of the substrate, reaching a piezoelectric device acting as receiving means. In this device, an AC electric field is produced between the electrode fingers of the IDT through a piezoelectric effect in the piezoelectric film. Induced thereby, an alternating current is outputted from the IDT of the device.
During operation of the SAW type touch panel device, surface acoustic waves are produced from each of the piezoelectric devices acting as exciting means, and these surface acoustic waves propagate through the detection region of the substrate, and are received by particular piezoelectric devices acting as receiving means. In the case that a finger or the like is in contact with the touch panel device in the detection region, the amplitude of surface acoustic waves passing through the position of contact will be attenuated. The position of contact in the detection region can be identified/detected by detecting and analyzing this attenuation.
With such a SAW type touch panel device, for the exciting piezoelectric devices, the higher the electromechanical conversion efficiency thereof, the more efficiently acoustic waves are excited for a given applied voltage. On the other hand, for the receiving piezoelectric devices, the higher the electromechanical conversion efficiency thereof, the more efficiently an alternating current is outputted based on the received acoustic waves. Consequently, with a SAW type touch panel device, the higher the electromechanical conversion efficiency of each of the piezoelectric devices, the smaller the insertion loss (dB) in the transmission and reception of a signal between a pair of piezoelectric devices, whereby the driving voltage of the touch panel device can be reduced, or the detection precision of the touch panel device can be increased.
However, with a conventional SAW type touch panel device, a sufficiently high electromechanical conversion efficiency is not obtained in the piezoelectric devices, and hence it may be that the driving voltage can not be sufficiently reduced, or the required detection precision cannot be obtained.